A new class of cardiac arrhythmias has been identified which result from a loss of functional ankyrin-B, a protein responsible for the targeting of several ion channels and transporters to specialized membrane domains. Ankyrin-B heterozygous mice exhibit EKG changes and arrhythmia similar to patients with ankyrin-B mutations. Ankyrin-B-/- cardiomyocytes have disrupted localization of IPS receptor (IP3R), Na+ /Ca2+ exchanger (NCX) and Na+/K+-ATPase (NKA) to sarcoplasmic reticulum/T-tubule junctions. The cellular pathway responsible for ankyrin-B-dependent organization of IP3R, NCX and NKA is unknown. Our laboratory recently determined that IP3R accumulates in an unknown compartment juxtaposed to Golgi in cardiomyocytes transfected with a dominant negative ankyrin-B mutant. Light and electron microscopy will be used to characterize this novel compartment. Other ankyrin-B mutants will then be examined for their ability to arrest the cellular distribution of these proteins and additional cellular intermediates will be characterized. The goal of this proposed research is to delineate the ankyrin-B dependent targeting pathways of IP3R, NCX and the NKA to SR/T-tubule junctions in cardiomyocytes.